Electromagnetic induction apparatus for high-voltage power generation

ABSTRACT

The power capability of insulating core-type transformers is greatly increased by creating additional magnetomotive force in certain secondary coils without any consequent power loss. At the same time, surge protection is provided for the primary power source without adversely affecting the auxiliary power source by using additional secondary cores with a resistive circuit isolating the high-voltage power supply from damaging highvoltage transient surges and providing artificial capacitance between the additional secondary cores and ground potential.

Y United States Patent Inventor Brian Skillicorn Topstield, Mass.

Appl. No. 833,436

Filed June 16, 1969 Patented Oct. 5, 1971 Assignee High VoltageEngineering Corporation Burlington, Mass.

ELECTROMAGNETIC INDUCTION APPARATUS FOR HIGH-VOLTAGE POWER GENERATION 16Claims, 3 Drawing Figs.

US. Cl 317/14, 3 36/ l 78 Int. Cl H07h 9/02 Field of Search 323/48;

[56] References Cited UNITED STATES PATENTS 3,187,208 6/1965 Van deGraaff 336/178 X Primary Examiner-James D. Trammell AssistantExaminer-Harry E. Moose, Jr. Attorneys-Irwin A. Shaw and Francis J.Thornton ABSTRACT: The power capability of insulating core-typetransformers is greatly increased by creating additional magnetomotiveforce in certain secondary coils without any consequent power loss. Atthe same time, surge protection is provided for the primary power sourcewithout adversely affecting the auxiliary power source by usingadditional secondary cores with a resistive circuit isolating thehigh-voltage power supply from damaging high-voltage transient surgesand providing artificial capacitance between the additional secondarycores and ground potential.

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INVENTOR BRIAN SKI LlC mg A TOR NEY P POWER SUPPLY PATENTEUUCT 519713611032 sum 2 UF 2 5 'IIIIIIIIIIIII'III"IIIIIIIIII'I" I I, I, l3

g I9 I I 8 'IIIIIIIIIIIIIIIII[III/IIIIIIIIIIIIIIIIII/IIIIIII l6 E3 E5III/I.IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII INVENTOR BRIANSKILLICO N BY &) Z A TORNEY ELECTROMAGNETIC INDUCTION APPARATUS FORHIGH-VOLTAGE POWER GENERATION BACKGROUND OF THE INVENTION High-voltagepower supplies of the type which are the subject of this invention areused, for example, to supply the acceleration voltage to chargedparticle accelerators used extensively in science and industry. Typicalapplications of such particle accelerators include nuclear research,industrial radiation processing, X-ray generators and electronmicroscopes. Charged particle accelerators typically comprise ahigh-voltage power supply connected to an evacuated acceleration tube.The source of the charged particles is located at the high-voltage endof the acceleration tube and it requires electrical energy to energizevarious accessory power supplies associated with the generation andfocusing of the charged particle beam. Existing techniques for supplyingthis auxiliary source of electrical energy include the use of isolationtransformers with high-voltage insulation between the primary andsecondary windings, mechanical couplings such as insulated shafts orbelts which turn a generator located in the high-voltage area, anddual-function power supplies of the insulating core transformer, orother inductively coupled types, in which a single unit generates boththe high acceleration voltage and the auxiliary source of electricalenergy. Charged particle accelerators are subject to occasional sparkbreakdown between the high-voltage terminal and ground resulting inhigh-voltage surges which may overstress parts of the power supply withconsequent component failure. These damaging voltage surges can begreatly reduced in their magnitude by the insertion of an impedance,such as a resistor, in the connecting link between the power supply andthe acceleration tube. In the event of a spark in the acceleration tube,the energy associated with the resulting voltage surge is then largelydissipated across the series impedance and the power supply is protectedfrom the surge voltage. However, surge-limiting impedances of thedescribed type cannot be used with dual-purpose power supplies whichsupply auxiliary electrical energy as well as the high acceleratingvoltage because the voltage drop caused by the series impedance, whilenegligible in comparison with the high voltage, would be an unacceptablyhigh proportion of the low auxiliary source voltage.

The conventional insulating core transformer has previously had thedisadvantage of nonuniformity of the magnetic field particularly as thenumber of secondary cores increases. The magnetic field linesoriginating in the primary cores begin to take a leakage path ratherthan through the intended magnetic circuit thereby decreasing the totalpower output that would otherwise be available if all the magnetic fluxcould be con trolled.

SUMMARY OF THE INVENTION This invention relates to the prevention ofdamage to highvoltage power supplies when subjected to voltage surgessuch as may occur when sparking takes place in an externally connectedload. In particular, this invention is concerned with the prevention ofsurge damage to power supplies which are used for the dual purpose ofsimultaneously generating a high voltage and an auxiliary lower voltage,this lower voltage being insulated from ground by the full value of thehigh voltage. Included in the scope of this invention is a techniquewhereby the power capability of transformers of the insulating core typecan be greatly increased in comparison with the ratings of suchtransformers built by conventional methods.

lt is, therefore, a general object of the present invention to provide anew and improved electromagnetic induction apparatus.

Another object of the present invention is to provide a new and improvedsurge protection system for power supplies of the multiplier type whereenergy is supplied to a plurality of cascaded voltage generators by analternating magnetic field.

Yet another object of the present invention is to increase the poweroutput and the efficiency of the magnetic circuit in electromagnetinduction apparatus.

The invention can be used with any power supply of the voltagemultiplier type where energy is supplied to a plurality of cascadedvoltage generators by an alternating magnetic field. A capacitanceconnected in parallel to' the secondary coils is used to increase theuniformity of the magnetic field distribution in an insulating coretransformer thus greatly increasing the power capability of thetransformer. The application of this refinement is not restricted toinsulating core transformers using an integral surge-limiting impedanceand auxiliary power source only, but maybe gsed in insulating coretransformers without either surge impedance or provisions for anauxiliary power source where the purpose is only to increase the powercapability of the insulating core transformer.

DESCRIPTION OF THE DRAWINGS The invention can best be understood fromthe following description by reference to the following drawings inwhich:

FIG. 1 is a schematic representation of a three-phase power supply forthe production of a high-voltage DC simultaneously with a source ofthree-phase AC isolated from ground at the DC potential including fluxequalization devices in accordance with the principles of the presentinvention but not including the surge-limiting impedance.

FIG. 2 is a schematic representation of a three-phase power supply forthe production of a high-voltage DC simultaneously with a source ofthree-phase AC isolated from ground at the DC potential including surgeimpedance limiting circuitry with magnetic flux equalizing circuitry inaccordance with the principles of the present invention.

FIG. 3 is a simplification showing the equivalent circuit of a powersupply incorporating this invention in order that its method ofoperation when subjected to transient voltage surges may better bedescribed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I, athree-phase insulating core transformer power supply is shown. Themagnetic circuit comprises three identical ferromagnetic primary cores 1interconnected by a ferromagnetic return yoke 2. A plurality offerromagnetic secondary cores 3 are stacked on the primary cores to formthree identical columns. The secondary cores 3 are insulated from oneanother, the primary cores and the upper return cores 5 by sheets ofinsulation material 4. The magnetic circuit is completed by an upperferromagnetic return yoke 6. Primary coils 7 surround each primary coreI and an alternating magnetic field is established in the elements ofthe magnetic circuit by supplying a three-phase alternating current tothe three primary coils 7. Each secondary core 3 is surrounded by asecondary coil 8 in which an alternating voltage is induced due to thealternating magnetic field in the secondary cores 3. Typically thisalternating voltage is rectified by a voltage doubler circuit 20consisting of rectifiers 9, current-limiting resistors 10 and capacitors11. The direct current outputs of the voltage doubler circuits 20 areconnected in series so that their individual outputs add up to give ahigh direct voltage which appears across the output terminals 12 and 13.If terminal 12 is at, or near, ground potential then the terminal 13will be the high-voltage output connection to which the accelerationtube or other high-voltage load is connected. An important feature ofthe insulating core transformer consists of the connection of eachsecondary core 3 to one end of its associated secondary coil 8 thusensuring a constant voltage gradient in each sheet of insulation. Threeadditional seconds ry coils 14 provide a source of auxiliary three-phasepower at the high DC potential with respect to ground. These three coilsare arranged in a Y-connection with the output terminals 15 referencedto the high-voltage DC output 13 as the threephase system neutralconnection. Capacitors l6, l7 and 18 are associated with aflux-equalizing refinement and their function will be described later.

Insulating core transformer power supplies of the type described abovehave been manufactured and operated successfully at output voltages upto 750 kv. DC. Attempts to build such'power supplies for higher voltageratings have revealed a weakness inherent in most forms of high-voltagepower supplies; namely, under conditions in which the externallyconnected load sparks, the resulting surge gives rise to a nonuniformvoltage gradient among the components comprising the power supply. Thisnonuniformity is caused by various capacities and inductances, bothintentional and unintentional between the various components whichdictate the transient voltage distribution during the time of the surge.This phenomenon is well known to those skilled in the art and it can beshown by a theoretical analysis that the maximum value of this transientvoltage gradient will appear across components at the high-voltage endof the power supply. In highvoltage power supplies of the type shown inFIG. 1, this transient condition leads to a breakdown of the insulationsheet nearest to the high-voltage output connection. From the foregoingit must not be construed that this effect does not occur at voltagesbelow 750 kv.; rather, the magnitude of the transient voltage surgeimpressed upon the upper sheet of insulation is not great enough tocause failure of that sheet in insulating core transformer powersupplies designed for operation at 750 kv. and below.

These transient voltages can be isolated from the power supply byinclusion of an electrical impedance connected between the power supplyand the load. However, such an impedance has an undesirable effect onthe auxiliary power source as has been described previously. The mannerin which this invention seeks to avoid these difficulties is bestunderstood by a consideration of FIG. 2 which shows the power supply ofFIG. 1 modified by the addition of one or more secondary cores 3, sheetsof insulation 4, secondary coils 8', capacitors l6 and resistors, 19.The purpose of coils 8, and capacitors 16 can be ignored at this time.

In the event that a spark or surge takes place in apparatus connected tothe high-voltage connection 13, it is intended that most of the surgevoltage will appear across the resistors 19 and that the energyassociated with the surge will be therein dissipated. The active part ofthe high-voltage power supply will therefore be protected from damaginghigh-voltage transient surges. The number of additional cores, sheets ofinsulation and resistors used will depend upon the voltage rating of thepower supply. The impulse voltage withstand capability of both theinsulation sheets 4' and the series-connected resistors 19 must be equalto the full-rated voltage of the power supply since the peak value ofthe transient voltage surge to which these components will be subjectedis approximately equal to the full-rated power supply voltage. Theauxiliary three-phase AC power source is largely unaffected by theadditional surge-isolating components since the magnetic flux ismaintained in the upper secondary cores independently of the surgeresistors.

The simple form of three-phase insulating core transformer power supplyas just described and shown in FIG. 2 exhibits an effect which isdetrimental to its performance if large currents are to be obtained athigh-voltage outputs. This effect manifests itself by a lower magneticflux density in those secondary cores furthest away from the primarycoils 7. The cause of this effect is inherent in the nature of theinsulating core principle and is due to the presence of the insulatingsheets 4 which introduce gaps of high magnetic reluctance between thesecondary cores 3. In power supplies with many secondary cores, thetotal magnetic reluctance of each of the three legs of the magneticcircuit becomes comparable with the reluctance between the legs with theconsequence that a considerable magnetic leakage path exists between thethree legs. Thus those magnetic field lines originating in the primarycores do not all reach the uppermost cores in each leg butpreferentially travel along the leakage path. In a typical case for aninsulating core transformer power supply rated at 3 million volts, itwas found that the magnetic flux density in the uppermost secondarycores was only one-third of that in each primary core. It will beunderstood that the leakage flux will be even greater if additionalsecondary cores with their associated insulation gaps areincorporated inorder to install a surge-limiting impedance of the type which is thesubject of this invention. The existence of such a substantial leakageflux reduces the power output capability of a given size of mag-.

netic circuit when compared with a similar circuit without leakage.

By the application of another novel technique, greatly increased powermay be obtained from an insulating transformer power supply even whenthe number of insulation gaps 4 and secondary cores 3 are made large inorder to incorporate a surge protection impedance. Referring again toFIG. 1 which schematically shows an insulating core transformer powersupply without surge protection impedance, the magnetic flux density canbe made the same in every secondary core by the addition of capacitorsl6, l7 and 18 across each respective transformer coil. These capacitorsl6, l7 and 18 will cause a higher current to flow in each respectivecoil without dissipating any additional power other than an increasedresistance loss in each coil and the dissipation loss in each respectivecapacitor. These additional losses are trivial when compared with theadvantages of such a system inasmuch as the additional current flowingin each respective coil provides a magnetomotive force which serves todrive the magnetic field through the insulation gap 4 adjacent to thecoil in question. It is, therefore, possible to select values ofcapacitors which will cause that amount of additional current to flow ineach coil to give uniform flux density throughout the magnetic circuit.

It is expected that some decay in flux density will occur in the uppersections of the insulating core transformer power supply when a currentis drawn from it. This is caused by the demagnetizing effect of the loadcurrent flowing in the coils and some degree of compensation for thiscan be provided by intentionally choosing different values forcapacitors l6 and 17 such that the magnetic flux density in the uppersecondary cores 5 is higher than that in the lower cores when no currentis drawn from the power supply. This technique of flux equalization asapplied to insulating core transformers will hereafter be referred to asthe continuous excitation principle. In some cases, it may not benecessary to provide capacitors across every coil and a satisfactoryapproximation to continuous excitation may be made by connectingcapacitors across a few coils only.

Referring again to FIG. 2, the continuous excitation principle can beapplied to an insulating core transformer using an integralsurge-protecting impedance by providing secondary coils 8 withcapacitors 16' whose sole purpose is to provide a localizedmagnetomotive force in the vicinity of the insulation gaps 4. There areno external connections to the coils 8' other than those required tointerconnect between the three phases and the secondary cores in orderto establish the DC potential of the subject cores, coils andcapacitors.

A further feature of the invention enables the surge protectionresistors to fulfill their purpose in a more efficient manner. Thisfeature may best be understood by reference to FIG. 3 in conjunctionwith FIG. 2. In FIG. 3 that part of the device illustrated in FIG. 2which generates the high voltage is represented as a power supply Pacross the output of which is an equivalent capacitor C. The surgeprotection impedances, 19 in FIG. 2, shown as two resistors R1 and R2are each shunted by a capacitor of value Cl and C2 respectively. Thesecapacitors are primarily formed by the capacitance between adjacentsecondary cores 3' in FIG. 2. Capacitors C3, C4 and C5 are the straycapacitances between the components associated with the surge protectionimpedances and any neighboring grounded structure such as a containingvessel. The function of both R1 and R2 as has been previously explainedis to isolate the power supply P from transient voltages generated bythe connected load. It can be shown that for maximum effectiveness of RIand R2 in performing their surge protection function, capacitors C, C3,C4 and C5 should be made as large as possible while capacitors Cl and C2should be made as small as possible. The refinement to the basicinvention seeks to increase the surge protection ability of R1 and R2 byincreasing the capacitances C, C3, C4 and C5 while decreasing thecapacitances C1 and C2. For example, the insulation sheets 4' shown inFIG. 2 can be made thicker, than the insulation sheets 4 in thevoltage-generating part of the power supply. Moreover, since the mainconcern is to make the series combination of Cl and C2 much lower invalue than C, the dielectric constants of the materials chosen for theinsulation sheets can be different. For example,

polyester film sold under the proprietary name Mylar has a dielectricconstant about 1.5 times that of polyethylene while both materials havegreat dielectric strength. It is, therefore, beneficial to use Mylar forinsulation sheets 4 and thicker pieces of polyethylene for insulationsheets 4' so that C l and C2 are made small while C is made as large aspossible. As a further feature of the invention, capacitors C3, C4 andC5 can be increased by making cores 3' in F IG. 2 larger than cores 3 sothat the area exposed to grounded structures is increased resulting in aproportional increase in capacity. Conducting electrodes known asequipotential rings (not shown) are sometimes used to control theelectric field gradient in the space between the power supply and itscontainer. Typically,

these equipotential rings are electrically connected to cores 3 and 3'so that further increase in capacitors C3, C4 and C5 is possible byusing wider equipotential rings in conjunction with larger cores 3'.

Nothing in the foregoing description is meant to imply any restrictionof this invention to use with either three-phase insulating coretransformers or to DC power supplies. The surge-protecting impedance canbe incorporated in any power supply either AC or DC in which a varyingmagnetic field is used to induce a voltage in coils which areelectrically insulated one from the other and then connected in seriesto produce a high voltage. Moreover, this invention is not limited topower supplies using ferromagnetic cores. The principle of continuousexcitation can be applied to insulating core transformers of any numberof phases and intended to produce either AC or DC outputs.

I claim:

1. Electromagnetic induction apparatus for the simultaneous generationof high-voltage electrical power together with a lower voltage auxiliarypower comprising:

a. a highvoltage output terminal,

b. a high-voltage intermediate terminal,

c. a low-voltage terminal,

d. first electrical insulating means for insulating said highvoltageintermediate terminal from said low-voltage terminal,

e. at least one auxiliary terminal electrically insulated from saidlow-voltage terminal,

f. means for creating a varying magnetic field,

g. a plurality of first electrical windings interposed between saidhigh-voltage output and low-voltage terminals and coupled by saidmagnetic field,

h. means for electrically connecting said first electrical windings tocreate a high voltage at said high-voltage intermediate terminal,

. at least one second electrical winding interposed between saidhigh-voltage output and auxiliary terminal and coupled by said magneticfield,

j. second insulating means for insulating said second electrical windingfrom said high-voltage intermediate terminal; and

k. electrical impedance means connected between said high-voltage outputterminal and said high-voltage intermediate terminal for providing surgeprotection for said high-voltage electrical power portion of saidelectromagnetic induction apparatus.

2. The electrical induction apparatus as set forth in claim 1 whereinsaid second insulating means minimizes the capacitance between saidhigh-voltage intermediate terminal and said high-voltage output termina3. The electrical induction apparatus as set forth in claim 2 whereinsaid capacitance is minimized by increasing the thickness of theinsulation.

4. The electrical induction apparatus as set forth in claim 2 whereinsaid capacitance is minimized by providing insulation having a lowerdielectric constant.

5. The electrical induction apparatus'as'set forth in claim 1 whereinsaid first insulating means maximizes the capacitance between saidhigh-voltage intermediate and said low-voltage terminal. t Y

6. The electrical induction apparatus as set forth in claim 2 whereinsaid first insulating means maximizes the capacitance between saidhigh-voltage intermediate and said low-voltage terminal. 7

7. The electrical induction apparatus as set forth in claim 1 whereinsaid apparatus further includes means for controlling the electricalfield gradient by providing equipotential rings electrically connectedto said first and second electrical windings and to said electricalimpedance means in a predetermined manner and wherein the ringsassociated with said second electrical windings and said electricalimpedance means have an increased surface area to maximize thecapacitance to said low-voltage terminal.

8. Insulating core induction apparatus comprising:

a. a high-voltage terminal,

b. a low-voltage terminal,

c. at least one magnetic circuit interposed between said high and lowvoltage terminals,

d. at least one magnetic column included in said magnetic circuit, saidcolumn comprising a plurality of magnetic core segments electricallyinsulated one from the other,

e. means for generating a varying magnetic field in said magneticcircuit,

f. a plurality of electrical windings interposed between saidhigh-voltage output and low-voltage terminals and coupled by saidmagnetic field and surrounding said magnetic core segments,

g. means for electrically connecting said electrical windings to createa high voltage at said high-voltage terminal; and

h. at least one capacitive element connected in parallel with at leastone of said electrical windings.

9. The The insulating core induction apparatus as set forth in claim 8having an auxiliary power source and wherein said apparatus furtherincludes a. a high-voltage intermediate terminal at the same potentialas said high-voltage output terminal,

b. a high-voltage output terminal,

c. at least one auxiliary terminal electrically insulated from saidlow-voltage terminal,

d. at least one auxiliary electrical winding interposed between saidhigh-voltage output and auxiliary terminal and surrounding one of saidmagnetic core segments; and

e. electrical impedance means connected between said high-voltage outputterminal and said high-voltage intermediate terminal for providing surgeprotection for said high-voltage electrical power portion of saidelectromagnetic induction apparatus.

10. The insulating core induction apparatus as set forth in claim 9wherein said magnetic column is extended to include additionalelectrically insulated magnetic core segments with said electricalimpedance means electrically connected in a predetermined manner betweensaid additional core segments.

1]. The insulating core induction apparatus as set forth in claim 10wherein said apparatus further includes a. flux-equalizing electricalwindings surrounding said additional electrically insulated magneticcore segments; and

b. a capacitive element electrically connected respectively in parallelwith each of said flux-equalizing electrical windings.

12. The insulating core induction apparatus as set forth in claim 10wherein said additional magnetic core elements are elongated to increasethe stray electrical capacitance of said thickness of the insulation.

15. The electrical induction apparatus as set forth in claim 10 whereinsaid capacitance is minimized by providing insulation having a lowerdielectric constant.

16. The insulating core induction apparatus as set forth in claim 10wherein the insulation material between said core segments is selectedto have a higher dielectric constant than the insulation materialbetween said additional magnetic core segments to maximize thecapacitance between the said highvoltage intermediate terminal and saidlow-voltage terminal.

1. Electromagnetic induction apparatus for the simultaneous generationof high-voltage electrical power together with a lower voltage auxiliarypower comprising: a. a high-voltage output terminal, b. a high-voltageintermediate terminal, c. a low-voltage terminal, d. first electricalinsulating means for insulating said highvoltage intermediate terminalfrom said low-voltage terminal, e. at least one auxiliary terminalelectrically insulated from said low-voltage terminal, f. means forcreating a varying magnetic field, g. a plurality of first electricalwindings interposed between said high-voltage output and low-voltageterminals and coupled by said magnetic field, h. means for electricallyconnecting said first electrical windings to create a high voltage atsaid high-voltage intermediate terminal, i. at least one secondelectrical winding interposed between said high-voltage output andauxiliary terminal and coupled by said magnetic field, j. secondinsulating means for insulating said second electrical winding from saidhigh-voltage intermediate terminal; and k. electrical impedance meansconnected between said highvoltage output terminal and said high-voltageintermediate terminal for providing surge protection for saidhigh-voltage electrical power portion of said electromagnetic inductionapparatus.
 2. The electrical induction apparatus as set forth in claim 1wherein said second insulating means minimizes the capacitance betweensaid high-voltage intermediate terminal and said high-voltage outputterminal.
 3. The electrical induction apparatus as set forth in claim 2wherein said capacitance is minimized by increasing the thickness of theinsulation.
 4. The electrical induction apparatus as set forth in claim2 wherein said capacitance is minimized by providing insulation having alower dielectric constant.
 5. The electrical induction apparatus as setforth in claim 1 wherein said first insulating means maximizes thecapacitance between said high-voltage intermediate and said low-voltageterminal.
 6. The electrical induction apparatus as set forth in claim 2wherein said first insulating means maximizes the capacitance betweensaid high-voltage intermediate and said low-voltage terminal.
 7. Theelectrical induction apparatus as set forth in claim 1 wherein saidapparatus further includes means for controlling the electrical fieldgradient by providing equipotential rings electrically connected to saidfirst and second electrical windings and to said electrical impedancemeans in a predetermined manner and wherein the rings associated withsaid second electrical windings and said electrical impedance means havean increased surface area to maximize the capacitance to saidlow-voltage terminal.
 8. Insulating core induction apparatus comprising:a. a high-voltage terminal, b. a low-voltage terminal, c. at least onemagnetic circuit interposed between said high and low voltage terminals,d. at least one magnetic column included in said magnetic circuit, saidcolumn comprising a plurality of magnetic core segments electricallyinsulated one from the other, e. means for generating a varying magneticfield in said magnetic circuit, f. a plurality of electrical windingsinterposed between said high-voltage output and low-voltage terminalsand coupled by said magnetic field and surrounding said magnetic coresegments, g. means for electrically connecting said electrical windingsto create a high voltage at said high-voltage terminal; and h. at leastone capacitive element connected in parallel with at least one of saidelectricaL windings.
 9. The The insulating core induction apparatus asset forth in claim 8 having an auxiliary power source and wherein saidapparatus further includes a. a high-voltage intermediate terminal atthe same potential as said high-voltage output terminal, b. ahigh-voltage output terminal, c. at least one auxiliary terminalelectrically insulated from said low-voltage terminal, d. at least oneauxiliary electrical winding interposed between said high-voltage outputand auxiliary terminal and surrounding one of said magnetic coresegments; and e. electrical impedance means connected between saidhigh-voltage output terminal and said high-voltage intermediate terminalfor providing surge protection for said high-voltage electrical powerportion of said electromagnetic induction apparatus.
 10. The insulatingcore induction apparatus as set forth in claim 9 wherein said magneticcolumn is extended to include additional electrically insulated magneticcore segments with said electrical impedance means electricallyconnected in a predetermined manner between said additional coresegments.
 11. The insulating core induction apparatus as set forth inclaim 10 wherein said apparatus further includes a. flux-equalizingelectrical windings surrounding said additional electrically insulatedmagnetic core segments; and b. a capacitive element electricallyconnected respectively in parallel with each of said flux-equalizingelectrical windings.
 12. The insulating core induction apparatus as setforth in claim 10 wherein said additional magnetic core elements areelongated to increase the stray electrical capacitance of saidadditional magnetic core elements to said low-voltage terminal.
 13. Theinsulating core induction apparatus as set forth in claim 10 whereinsaid apparatus further includes means for controlling the electric fieldgradient by providing equipotential rings electrically connected to saidmagnetic core segments and wherein the rings associated with saidadditional magnetic core segments have an increased surface area tomaximize the capacitance to said low-voltage terminal.
 14. Theelectrical induction apparatus as set forth in claim 10 wherein saidcapacitance is minimized by increasing the thickness of the insulation.15. The electrical induction apparatus as set forth in claim 10 whereinsaid capacitance is minimized by providing insulation having a lowerdielectric constant.
 16. The insulating core induction apparatus as setforth in claim 10 wherein the insulation material between said coresegments is selected to have a higher dielectric constant than theinsulation material between said additional magnetic core segments tomaximize the capacitance between the said high-voltage intermediateterminal and said low-voltage terminal.